ACM Front Panel

Working our way down the front panel, we have the following LEDs and connectors:

Dedicated Inputs

• N-LED indicates when the ACM is being addressed via the CAMAC dataway
• CLK input is the 50 MHz TTL square wave input, derived from the 5× multiplied 10 MHz output of our GPS-disciplined clock.
• 1_PPS input is the One Pulse-Per-Second output from the GPS clock, marking the exact beginning of each second (phase-locked to the 50 MHz). The associated (green) LED indicates a "pulse".
• PIN input is from the fast photodiode. This post-discrimination 100 ns TTL pulse indicates that the laser has fired.
• CAL input is an optional input requesting an APD gate-open. Ordinarily, this signal is generated from the PIN signal.
• IDX is the index input from the motor driving the T/R switch rotating optic. This announces the phase of the mirror in its rotation.
• ENC is the encoder input from the T/R switch motor, with 2000 TTL pulses per cycle. In combination with IDX, this signal informs the ACM when to fire the laser so that the laser hits the reflective patch on the rotating optic.

Dedicated Outputs

• LUN_ST is a 20-ns TTL whose rising edge is coincident with the turn-on of the APD gate associated with the lunar photon. This output is actually used to request TDC START events for the purpose of calibration.
• CAL_STis a 20-ns TTL whose rising edge is coincident with the turn-on of the APD gate associated with the laser fire (either via PIN or CAL prompts).
• OPN is a generic gate-open announcement, and represents the always-enabled OR combination of the previous two signals--any time the gate opens for any purpose.
• CLS is a 20-ns TTL announcing the gate is about to close. The gate shuts off with the trailing edge of this pulse. Thus OPN and CLS are nestled inside the gate pulse (which is a multiple of 20 ns long). See the example ACM waveform for a picture.
• STOP is the same as CLS, but specifically intended to request a STOP pulse for the TDC. Given propagation delays, the first clock pulse following gate closure is selected.
• TME is a 160-ns TTL sent to the XL-DC GPS clock upon the first CLS event within a given second (as announced by the 1_PPS signal). This signal latches the GPS time to the nearest microsecond.
• GTE is a TTL signal of variable duration (a multiple of 20 ns) coincident with the request for APD gate action. See the example ACM waveform for a picture. The associated (white) LED pulses with the gate (stretched).
• APD is an enabled version of GTE, and passes the gate request to the APD. The associated (yellow) LED indicates that the APD is enabled.
• LSR is a DC signal indicating that the laser is enabled. This signal also tells the laser to charge its capacitor bank. The associated (green) LED announces the high voltage state of this line.
• ZAP is a 50 µs TTL pulse requesting a fire event from the laser. The associated (blue) LED pulses with the fire request.
• BLK is a DC signal that requests blockage of the laser beam. Only if the voltage is high will the beam be unblocked. A red LED indicates when the beam is un-blocked.

Configurable Inputs/Outputs

These five connectors (I/O 1 through I/O 5) are jumper-configurable to act either as inputs to the TIMER chip, or can be outputs. One likely use will be to activate additional shutters or actuate deployable corner-cube prisms within the telescope.

APOLLO Command Module

APOLLO Apparatus Page

APOLLO Main Page